Certain types of waste water, such as waste water from the galvano-industry, the wood impregnation industry, the paint and lacquer industry, power plants and shipyards, as well as the ground water polluted with waste products from deposits of solid waste, contain considerable amounts of heavy metals.
In conventional mechanical/biological or chemical purification of heavy metal-containing waste water the major portion of such heavy metals can be removed by precipitation, but the known purification methods result in the formation of sludge with such high contents of heavy metals that the sludge cannot be disposed of in a usual manner. This leads to increased operation costs for conventional purification plants.
Other water types, such as oxygen-free ground water and acid waste water types from metal-machining industries, contain considerable amounts of dissolved ferrous iron (Fe.sup.2+).
Conventional treatment (purification) of ferrous iron-containing water types comprises addition of an oxidant, such as atmospheric air, pure oxygen, chlorine or hydrogen peroxide. Hereby trivalent iron (ferric iron, Fe.sup.3+) is formed which, due to its poor solubility in water, is precipitated in the form of iron oxyhydroxide which can be separated from the water phase by sedimentation and/or filtration. In order to improve the separation of the iron oxyhydroxide it is known to add a polyelectrolyte and/or inert carrier materials to the water.
Various alternative purification methods have been developed with the object of reducing the content of heavy metals in aqueous media of the type disclosed above to acceptable levels, i.e. to below 0.5 mg/l.
One such known purification method consists in the heavy metal-containing aqueous medium contacting an adsorbent of amorphous iron oxyhydroxide at a pH-value of 6-7 whereby the heavy metals contained in the aqueous medium are bonded to the surface of the amorphous iron oxyhydroxide from which they are subsequently removed by eluation so as to form highly concentrated solutions of the removed heavy metals, cf. M. Edwards and M. M. Benjamin: Adsorptive filtration using coated sand, a new approach for treatment of metal bearing wastes. J. WPCF, 61, (1989), pp. 1523-1533.
It is also known that precipitated iron oxyhydroxide has a catalytic effect on the oxidation of ferrous iron, cf. U. Hasselbarth and D. Lutermann: Die Biologische Enteisenung und Entmanganung vom Wasser, 38 (1971), pp. 233 and A. Hult: Filtration of Iron During and After Oxidation. Effluent and Water Treatment, J., 13, (1973), pp. 209-215. This catalytic effect is due to fact that ferrous iron is adsorbed onto the surface of iron oxyhydroxide and in this position it is oxidized to ferric iron which hydrolyses to iron oxyhydroxide, cf. H. Tamura, K. Goto and M. Nagayama: The Effect of Ferric Hydroxide on the Oxygenation of Ferrous Ions in Neutral Solutions. Corrosion Science, 16 (1976), pp. 197-207 and W. Sung and J. J. Morgan: Kinetics and Product of Ferrous Iron Oxygenation in Aqueous Systems. Environ. Sci. & Technol., 14, (1980), pp. 561-568.
JP patent publication No. 59-016590 discloses a method for the purification of heavy metal-containing waste water. In this method magnetite sludge is added to the waste water, the particles of said sludge being coated with an iron oxyhydroxide layer. A base is subsequently added and the mixture is stirred. Hereby the heavy metals are adsorbed onto the iron oxyhydroxide-coated magnetite particles. Following precipitation of the particles thus obtained, the major portion (about 80%) of the water is separated off and the remaining portion as well as the precipitated particles are subjected to various treatments whereby magnetite and ferrite particles coated with a surface layer of iron oxyhydroxide are formed. Following separation of said particles from the aqueous phase, the particles are suitable for use in a renewed water purification operation.
It is the object of the present invention to provide a continuous method for removing metals and in particular heavy metals from aqueous media containing the same. It is also the object of the invention to facilitate the separation of the metal-coated carrier material particles from the purified aqueous medium.